Signals are affected by dispersion during transmission in an optical fiber. The so-called dispersion refers to waveform distortion of signals caused by different frequency components of the transmitted signal or different transmission speeds of various mode components of signals. The main influence on optical transmission caused by dispersion is intersymbol interference generated between data pulses, and the damage on the system performance caused by dispersion usually cannot be ignored. Therefore, an optical fiber transmission system having a transmission speed higher than 10 Gbit/s generally needs a dispersion compensation technology to ensure a transmission function of the system. Electronics dispersion compensation, especially electronic dispersion pre-compensation (electronic pre distortion, EPD) performed at an originating, has long-distance dispersion compensation capability. As shown in FIG. 1, in an EPD system, a digital signal processor (DSP) sends two signals, the two signals are multiplexed by a multiplexer, the multiplexed signals are converted by a digital-to-analog converter into analog signals, and the analog signals are amplified by an amplifier, so that two signals I and Q are modulated onto a light signal sent by a light source for transmission.
However, due to components and wiring in the circuit, the two signals I and Q usually have a delay difference. The delay difference between the upper signal and the lower signal greatly influences the performance of the EPD system. It is found through practical tests that when the deviation exceeds 0.2 times of the 1/bit cycle, a bit error rate is increased from 10−6 to about 10−4. FIG. 2 shows another EPD system in the prior art, in which a phase shifter is connected before the multiplexer to change the delay difference between the two signals I and Q by changing phases.
The inventor finds through in-depth study that in the prior art, the delay difference must be processed in an offline state (that is, when the delay difference is adjusted, no service can be transmitted), the operation is complicated, and real-time monitoring and adjustment of the delay cannot be achieved.
The same problem exists not only in the EPD system but also in other systems in which the delay difference needs to be adjusted.